Aha, thanks! I forgot all about the "Show remote" button in Modular III which is almost exactly what I was after!
Seems like it only supports dials, not switches or faders, and is non-configurable in terms of layout.

Any other options that offer switches, faders, and more configurability?

(You might be able to tell that I've been inspired by Faxi Nadu's videos )

We have a VST version released to our beta testers, but that isn't user-customisable at this stage (although we will be releasing it as open source, if you don't mind getting your hands a bit dirtier!) Get in touch if you'd like to be involved in beta testing.

P.S. Even without using ScopeSync, if you just want to make cut-down control interfaces for modular, you should definitely look at the rest of the BC Modular package, especially the BCM Modules (wiki descriptions here: BCM Modules). Don't be fooled by the GUIs, these don't require any specific hardware.